Electrocardiography systems serve to monitor the electrical activity of the heart by recording and processing electrical signals measured with a number of electrodes placed on a patient's body. An electrocardiography system generally includes, in addition to the (one or more) electrodes, a processing device (e.g., a general-purpose computer running suitable software) connected to the electrodes via a suitable cable.
In conventional electrocardiography systems, the electrical signals for a number of leads (that is, electrodes or combinations thereof) are displayed on-screen or printed, in the form of time-varying waveforms called electrocardiograms, for interpretation by a clinician. The electrocardiograms usually exhibit a periodicity corresponding to the patient's heart beat and, within the signal portion for each cardiac cycle, characteristic features (such as peaks, decays, waves, etc., including, e.g., the QRS complex and T wave) corresponding to various physiological processes, such as depolarization and repolarization of the ventricles of the heart. More recently developed advanced electrocardiography systems moreover process the electrocardiograms, using new signal-processing techniques and algorithms, to obtain, e.g., time-frequency maps of the electrical activity or quantitative metrics of heart condition and health that increase the diagnostic potential of electrocardiography.
Most conventional electrocardiography systems measure signals in the millivolt range, as the entire amplitude of a QRS complex can be seen on a +/−5 mV scale (and most systems display the signals on a +/−2 mV scale). Signals less than 0.1 mV are typically considered noise, and are undesirable for final electrocardiogram readings. In fact, most electrocardiography systems implement filtering to hide stand-alone signals of low voltages. As a result, accuracy in the lower voltages (corresponding to the least significant bits) of an acquired signal have not been an important design goal of these systems, and often cost considerations drive design decisions to adopt less accurate materials for the electrodes and cables. For most of these systems, that is a reasonable trade-off. The above-mentioned advanced electrocardiography systems, however, involve signal-processing techniques that are contingent upon a higher information content and accuracy of the incoming electrical signals, with noise levels as low as a few microvolts. Using conventional, off-the-shelf electrodes and cables in these systems can degrade the accuracy and performance of the advanced signal-processing techniques.